Epoxides are useful intermediates in organic synthesis because of their high reactivity. Among many chemical processes, cycloaddition reactions of epoxides with heterocumulenes are efficient methods for the synthesis of heterocyclic compounds like oxazolidinones. Oxazolidiones are widely used structural motifs in pharmaceutical applications and the cycloaddition of aromatic epoxides and isocyanates seems to be a convenient one-pot synthetic route to it. Expensive catalysts, reactive polar solvents, long reaction times and low selectivities are very common in early reports (M. E. Dyen and D. Swern, Chem. Rev., 67, 197 (1967); X. Zhang and W. Chen, Chem. Lett., 39, 527 (2010); M. T. Barros and A. M. F. Phillips, Tetrahedron: Asymmetry, 21, 2746 (2010); H.-Y. Wu, J.-C. Ding and Y.-K. Liu, J. Indian Chem. Soc., 80, 36 (2003); C. Qian and D. Zhu, Synlett, 129 (1994)). Therefore, the development of suitable catalyst systems and the identification of improved reaction conditions are desirable.
The catalytic reaction of diisocyanates with diepoxides leads to linear oligo- or polyoxazolidinones. In order for them to melt at elevated temperatures and to be useful as thermoplastics, the chemical conversion should have a high selectivity to the oxazolidinone.
With respect to catalysts, these are generally known for the reaction of isocyanates with epoxides. For example, the publication of H.-Y. Wu, J.-C. Ding and Y.-K. Liu, J. Indian Chem. Soc., 80, 36 (2003) discusses SmI3 as a catalyst. However, this reaction is run with 10 mol-% of catalyst. The publication of M. Fujiwara, A. Baba, Y. Tomohisa and Haruo Matsuda, Chem. Lett. 1963-1966 (1986) describes a catalyst system of Ph4SbI-Bu3SnI. The use of organotin compounds is disadvantageous due to their toxicity. Furthermore, the catalyst concentration of 5 mol-% is rather high. As the catalyst would remain in a final polymer product, using only low amounts of catalyst is the preferred path.
U.S. Pat. No. 3,471,442 is directed towards the preparation of thermoplastic polymers by heating a solution of a diepoxide monomer dissolved in an inert organic solvent at a temperature of at least 115° C., said solvent containing a catalytic quantity of an alkali metal alkoxide. An aromatic diisocyanate is added to the aforementioned solution in small increments over a period of about one hour and heating is continued after the incremental addition of the diisocyanate has been completed until the reaction between the diepoxide and the diisocyanate is substantially completed. Thereafter, the polymeric material is separated from the solvent.
WO 86/06734 discloses a method for producing a polyoxazolidinone compound from an epoxide and an isocyanate. The reaction can be run batch-wise or continuously. However, there is no disclosure in WO 86/06734 to add the isocyanate compound to the epoxide compound continuously or step-wise.
US 2010/227090 discloses a method for producing a polyoxazolidinone from an epoxide and an isocyanate. The reaction is catalysed by 2-phenyl imidazole. There is no disclosure to use Lewis acids as catalysts.
U.S. Pat. No. 3,020,262 discloses a method for producing 2-oxazolidinones from epoxides and isocyanates in the presence of trialkylamines, alkali metal halides and ammonium halides.
It would be desirable to identify reaction conditions, which lead to oxazolidinones and polyoxazolidinones with a higher selectivity, which employ less catalyst and which avoid the use of toxic (co-)catalysts. In accordance with a lower catalyst loading and a higher selectivity, it would also be desirable to have access to oligomeric or polymeric oxazolidinone compounds with a reduced discoloration.